Sexual reproduction is more prevalent in continental landscapes in the expanding arctic shrub, Salix glauca

The recent expansion of arctic deciduous shrubs has been well documented across a range of habitats, but the phenomenon is not universal. Their spread is often associated with increases in temperature and other abiotic factors, while variation in habitat moisture and herbivory can mediate the location and rate of this rise in abundance. Much less is known about the mode of increase of arctic shrubs. For one such shrub, Salix glauca, we used microsatellite markers to assess the prevalence of clonal growth (i.e. vegetative spread) and sexual reproduction (i.e. recruitment from seed) at sites with maritime and continental climates and differing in the density of large herbivores. We sampled individuals in plots reflecting the spatial scale of expansion in locations where S. glauca recently increased in abundance. The 400 samples collected across the four sites comprised 310 genotypes. Though evidence of sexual recruitment was common across all sites, coastal sites contained both more and larger clonal genotypes. While we expected soil conditions would be influential, the factors that best predicted the likelihood of clonality, genet size and vascular plant cover, suggest the light environment is of primary importance. Furthermore, in spite of the large distances between sites, there was no suggestion of genetic differentiation into distinct populations. These results indicate that differences in climate and herbivory can influence not only where and how extensively deciduous shrubs spread, but how they are likely to do so. We suggest future research integrating how mode of increase is associated with the rate of spread will advance projections of change in arctic ecosystems.